Proteolytic processing is an essential component of normal cell growth, differentiation, remodeling, and homeostasis. The cleavage of peptide bonds within cells is necessary for the maturation of precursor proteins to their active form, the removal of signal sequences from targeted proteins, the degradation of incorrectly folded proteins, and the controlled turnover of peptides within the cell. Proteases participate in apoptosis, inflammation, and in tissue remodeling during embryonic development, wound healing, and normal growth. They are necessary components of bacterial, parasitic, and viral invasion and replication within a host. Four principal categories of mammalian proteases have been identified based on active site structure, mechanism of action, and overall three-dimensional structure. (Beynon, R. J. and J. S. Bond (1994) Proteolytic Enzymes: A Practical Approach, Oxford University Press, New York, NY, pp. 1-5.)
The serine proteases (SPs) are a large family of proteolytic enzymes that include the digestive enzymes, trypsin and chymotrypsin; components of the complement cascade and of the blood-clotting cascade; and enzymes that control the degradation and turnover of macromolecules of the extracellular matrix. SPs are so named because of the presence of a serine residue found in the active catalytic site for protein cleavage and usually within the sequence GDSGGP. The active site of all SP is composed of a triad of residues including the aforementioned serine, an aspartate, and a histidine residue. SPs have a wide range of substrate specificities and can be subdivided into subfamilies on the basis of these specificities. The main sub-families are trypases which cleave after arginine or lysine; aspases which cleave after aspartate; chymases which cleave after phenylalanine or leucine; metases which cleavage after methionine; and serases which cleave after serine.
The SPs are secretory proteins containing N-terminal signal peptides which export the immature protein across the endoplasmic reticulum prior to cleavage. (von Heijne, G. (1986) Nuc. Acid. Res. 14:5683-5690). Differences in these signal sequences provide one means of distinguishing individual SPs. Some SPs, particularly the digestive enzymes, exist as inactive precursors or preproenzymes and contain a leader or activation peptide on the C-terminal side of the signal peptide. This activation peptide may be 2-12 amino acids in length, and extend from the cleavage site of the signal peptide to the N-terminus of the active, mature protein. Cleavage of this sequence activates the enzyme. This sequence varies in different SPs according to the biochemical pathway and/or its substrate. (Zunino, S. J. et al. (1990) J. Immunol. 144:2001-2009; and Sayers, T. J. et al. (1994) J. Immunol. 152:2289-2297.)
Cysteine proteases are involved in diverse cellular processes ranging from the processing of precursor proteins to intracellular degradation. Mammalian cysteine proteases include lysosomal cathepsins and cytosolic calcium activated proteases, calpains. Cysteine proteases are produced by monocytes, macrophages and other cells of the immune system which migrate to sites of inflammation and in their protective role secrete various molecules to repair damaged tissue. These cells may overproduce the same molecules and cause tissue destruction in certain disorders. In autoimmune diseases such as rheumatoid arthritis, the secretion of the cysteine protease, cathepsin C, degrades collagen, laminin, elastin and other structural proteins found in the extracellular matrix of bones. The cathepsin family of lysosomal proteases includes the cysteine proteases; cathepsins B, H, K, L, O2, and S; and the aspartyl proteases; cathepsins D and G. Various members of this endosomal protease family are differentially expressed. Some, such as cathepsin D, have a ubiquitous tissue distribution while others, such as cathepsin L, are found only in monocytes, macrophages, and other cells of the immune system.
Abnormal regulation and expression of cathepsins is evident in various inflammatory disease states. In cells isolated from inflamed synovia, the mRNA for stromelysin, cytokines, TIMP-1, cathepsin, gelatinase, and other molecules is preferentially expressed. Expression of cathepsins L and D is elevated in synovial tissues from patients with rheumatoid arthritis and osteoarthritis. Cathepsin L expression may also contribute to the influx of mononuclear cells which exacerbates the destruction of the rheumatoid synovium. (Keyszer, G. M. (1995) Arthritis Rheum. 38:976-984.) The increased expression and differential regulation of the cathepsins is linked to the metastatic potential of a variety of cancers and as such is of therapeutic and prognostic interest. (Chambers, A. F. et al. (1993) Crit. Rev. Oncog. 4:95-114.)
Cysteine proteases are characterized by a catalytic domain containing a triad of amino acid residues similar to that found in serine proteases. A cysteine replaces the active serine residue. Catalysis proceeds via a thiol ester intermediate and is facilitated by the side chains of the adjacent histidine and aspartate residues.
Aspartic proteases include bacterial penicillopepsin, mammalian pepsin, renin, chymosin, and certain fungal proteases. The characteristic active site residues of aspartic proteases are a pair of aspartic acid residues, e.g., asp33 and asp21 3 in penicillopepsin. Aspartic proteases are also called acid proteases because the optimum pH for activity is between 2 and 3. In this pH range, one of the aspartate residues is ionized, the other unionized. A potent inhibitor of aspartic proteases is the hexapeptide, pepstatin, which in the transition state resembles normal substrates.
Carboxypeptidases A and B are the principal mammalian representatives of the metallo-protease family. Both are exopeptidases of similar structure and active sites. Carboxypeptidase A, like chymotrypsin, prefers C-terminal aromatic and aliphatic side chains of hydrophobic nature, whereas carboxypeptidase B is directed toward basic arginine and lysine residues. Active site components include zinc, with its three ligands of two glutamic acid and one histidine residues.
Many other proteolytic enzymes do not fit any of the major categories discussed above because their mechanisms of action and/or active sites have not been elucidated. These include the aminopeptidases and signal peptidases.
Aminopeptidases catalyze the hydrolysis of amino acid residues from the amino terminus of peptide substrates. Bovine leucine aminopeptidase is a zinc metallo-enzyme that utilizes the sulfydryl groups from at least three reactive cysteine residues at its active site in the binding of metal ions. (Cuypers, H. T. et al. (1982) J. Biol. Chem. 257:7086-7091.)
Signal peptidases are a specialized class of proteases found in all prokaryotic and eukaryotic cell types that serve in the processing of signal peptides from certain proteins. Signal peptides are amino-terminal sequences on a protein which directs the protein from its ribosomal assembly site to a particular cellular or extracellular location. Once the protein has been exported, removal of the signal sequence by a signal peptidase and posttranslational processing, e.g., glycosylation or phosphorylation, activate the protein. Signal peptidases exist as multi-subunit complexes in both yeast and mammals. The canine signal peptidase complex is composed of five subunits; all associate with the mitochondrial membrane, and containing hydrophobic regions that span the membrane one or more times. (Shelness, G. S. and Blobel, G. (1990) J. Biol. Chem. 265:9512-9519.) Some of these subunits serve to fix the complex in its proper position on the membrane while others contain the actual catalytic activity. The catalytic activity appears to involve a serine residue in its active site.
Proteasome is an intracellular protease complex which is found in some bacteria and in all eukaryotic cells and plays an important role in cellular physiology. Proteasomes are responsible for the timely degradation of cellular proteins of all types and control proteins that function to activate or repress cellular processes such as transcription and cell cycle progression. (Ciechanover, A. (1994) Cell 79:13-21.) Proteasomes act on proteins which have been targeted for hydrolysis by the covalent attachment of a small protein called ubiquitin to lysine side chains of the protein. Ubiquitin-proteasome systems are implicated in the degradation of mitotic cyclic kinases, oncoproteins, tumor suppressor genes (p53), cell surface receptors associated with signal transduction, transcriptional regulators, and mutated or damaged proteins. (Ciechanover, supra.) Proteasomes are large (.about.2000 kDa), multisubunit complexes composed of a central catalytic core containing a variety of proteases and terminal subunits that serve in substrate recognition and regulation of proteasome activity.
Protease inhibitors play a major role in the regulation of the activity and effect of proteases. They have been shown to control pathogenesis in animal models of proteolytic disorders. (Murphy, G. (1991) Agents Actions Suppl 35:69-76.) In particular, low levels of the cystatins, low molecular weight inhibitors of the cysteine proteases, seem to be correlated with malignant progression of tumors. (Calkins, C. et al (1995) Biol Biochem Hoppe Seyler 376:71-80.) The balance between levels of cysteine proteases and their inhibitors is also significant in the development of disorders. Specifically, increases in cysteine protease levels, when accompanied by reductions in inhibitor activity, are correlated with increased malignant properties of tumor cells and the pathology of arthritis and immunological diseases in humans.
The serpins are high molecular weight, e.g., 370-420 amino acid residues, inhibitors of mammalian plasma serine proteases. Many of these inhibitors serve to regulate the blood clotting cascade and/or the complement cascade in mammals. Prominent among these inhibitors are .alpha.-1 protease inhibitor, .alpha.-1-antichymotrypsin, antithrombin III, and the "universal protease inhibitor" .alpha.-2 macroglobulin. .alpha.-1 protease inhibitor is primarily effective against the neutrophil elastase but combines with other serine proteases as well. .alpha.-1 protease inhibitor, .alpha.-1-antichymotrypsin, and antithrombin III all show striking sequence homology, suggesting that specialization of these inhibitors has occurred in response to specialization of the corresponding proteases themselves.
The discovery of new human protease molecules and the polynucleotides encoding them satisfies a need in the art by providing new compositions which are useful in the diagnosis, treatment, and prevention of cell proliferative and immune disorders.